Missing required arguments: aws_access_key_id, aws_secret_access_key (ArgumentError)

前提・実現したいこと

Ruby on Railsでwebシステムを作っています。
herokuでデプロイし、railsコンソールを確認すると以下のエラーメッセージが発生しました。

発生している問題・エラーメッセージ

Traceback (most recent call last):
        53: from /app/bin/rails:9:in `<main>'
        52: from /app/bin/rails:9:in `require'
        51: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/commands.rb:16:in `<top (required)>'
        50: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/command.rb:44:in `invoke'
        49: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/command/base.rb:63:in `perform'
        48: from /app/vendor/bundle/ruby/2.6.0/gems/thor-1.0.1/lib/thor.rb:392:in `dispatch'
        47: from /app/vendor/bundle/ruby/2.6.0/gems/thor-1.0.1/lib/thor/invocation.rb:127:in `invoke_command'
        46: from /app/vendor/bundle/ruby/2.6.0/gems/thor-1.0.1/lib/thor/command.rb:27:in `run'
        45: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/commands/console/console_command.rb:96:in `perform'
        44: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/command/actions.rb:16:in `require_application_and_environment!'
        43: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/application.rb:329:in `require_environment!'
        42: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/application.rb:329:in `require'
        41: from /app/config/environment.rb:5:in `<top (required)>'
        40: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/application.rb:353:in `initialize!'
        39: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/initializable.rb:58:in `run_initializers'
        38: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:205:in `tsort_each'
        37: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:226:in `tsort_each'
        36: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:347:in `each_strongly_connected_component'
        35: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:347:in `call'
        34: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:347:in `each'
        33: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:349:in `block in each_strongly_connected_component'
        32: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:415:in `each_strongly_connected_component_from'
        31: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:415:in `call'
        30: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/initializable.rb:48:in `tsort_each_child'
        29: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/initializable.rb:48:in `each'
        28: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:421:in `block in each_strongly_connected_component_from'
        27: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:431:in `each_strongly_connected_component_from'
        26: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:422:in `block (2 levels) in each_strongly_connected_component_from'
        25: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:350:in `block (2 levels) in each_strongly_connected_component'
        24: from /app/vendor/ruby-2.6.6/lib/ruby/2.6.0/tsort.rb:228:in `block in tsort_each'
        23: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/initializable.rb:59:in `block in run_initializers'
        22: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/initializable.rb:30:in `run'
        21: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/initializable.rb:30:in `instance_exec'
        20: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/engine.rb:611:in `block in <class:Engine>'
        19: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/engine.rb:611:in `each'
        18: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/engine.rb:612:in `block (2 levels) in <class:Engine>'
        17: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/engine.rb:654:in `load_config_initializer'
        16: from /app/vendor/bundle/ruby/2.6.0/gems/activesupport-5.1.7/lib/active_support/notifications.rb:168:in `instrument'
        15: from /app/vendor/bundle/ruby/2.6.0/gems/railties-5.1.7/lib/rails/engine.rb:655:in `block in load_config_initializer'
        14: from /app/vendor/bundle/ruby/2.6.0/gems/activesupport-5.1.7/lib/active_support/dependencies.rb:286:in `load'
        13: from /app/vendor/bundle/ruby/2.6.0/gems/activesupport-5.1.7/lib/active_support/dependencies.rb:258:in `load_dependency'
        12: from /app/vendor/bundle/ruby/2.6.0/gems/activesupport-5.1.7/lib/active_support/dependencies.rb:286:in `block in load'
        11: from /app/vendor/bundle/ruby/2.6.0/gems/activesupport-5.1.7/lib/active_support/dependencies.rb:286:in `load'
        10: from /app/config/initializers/carrier_wave.rb:2:in `<top (required)>'
         9: from /app/vendor/bundle/ruby/2.6.0/gems/carrierwave-1.2.2/lib/carrierwave.rb:14:in `configure'
         8: from /app/vendor/bundle/ruby/2.6.0/gems/carrierwave-1.2.2/lib/carrierwave/uploader/configuration.rb:159:in `configure'
         7: from /app/config/initializers/carrier_wave.rb:4:in `block in <top (required)>'
         6: from /app/vendor/bundle/ruby/2.6.0/gems/carrierwave-1.2.2/lib/carrierwave/uploader/configuration.rb:137:in `fog_credentials='
         5: from /app/vendor/bundle/ruby/2.6.0/gems/carrierwave-1.2.2/lib/carrierwave/uploader/configuration.rb:124:in `eager_load_fog'
         4: from /app/vendor/bundle/ruby/2.6.0/gems/fog-core-1.45.0/lib/fog/storage.rb:27:in `new'
         3: from /app/vendor/bundle/ruby/2.6.0/gems/fog-core-1.45.0/lib/fog/core/services_mixin.rb:16:in `new'
         2: from /app/vendor/bundle/ruby/2.6.0/gems/fog-core-1.45.0/lib/fog/core/service.rb:98:in `new'
         1: from /app/vendor/bundle/ruby/2.6.0/gems/fog-core-1.45.0/lib/fog/core/service.rb:268:in `handle_settings'
/app/vendor/bundle/ruby/2.6.0/gems/fog-core-1.45.0/lib/fog/core/service.rb:244:in `validate_options': Missing required arguments: aws_access_key_id, aws_secret_access_key (ArgumentError)

carrier_wave.rb

if Rails.env.production?
  CarrierWave.configure do |config|
    config.fog_provider = 'fog/aws'
    config.fog_credentials = {
      # Amazon S3用の設定
      :provider              => 'AWS',
      :region                => ENV['S3_REGION'], 
      :aws_access_key_id     => ENV['S3_ACCESS_KEY'],
      :aws_secret_access_key => ENV['S3_SECRET_KEY']
    }
    config.fog_directory     =  ENV['S3_BUCKET']

end

試したこと

git config で環境変数を確認しましたが、問題はありませんでした。

「Missing required arguments: aws_access_key_id, aws_secret_access_key」
で検索してヒットする解決方法は一通り試してますが、解決には至りませんでした。

何卒、よろしくお願い致します。

補足情報（FW/ツールのバージョンなど）

Ruby 2.6.3
Rails 5.1.7